Conventional I/Q modulators are used in transmitting means for carrier-frequency transmission systems, e.g. transmitters for digital broadcasting, and in base stations for mobile communications.
One example of such a transmitting means is shown in FIG. 4. The transmitting means 400 comprises an I/Q modulator 402 with predistortion, the I/Q modulator 402 comprising a first input connected to the input of the transmitting means 400, and an output. The first input of the I/Q modulator has an I/Q signal applied thereto. The output of the I/Q modulator is connected to a first input of a first mixer 404. A second input 404 of the mixer is connected to an oscillator 406. An output of the mixer 404 is connected to an input of an amplifier 408. An output of the amplifier 408 is connected to an antenna 410. The amplifier 408 and the antenna 410 have arranged between them a decoupling means 412 which is connected to the input of an attenuator 414. An output of the attenuator 414 is connected to a first input of a second mixer 416. A second input of the second mixer 416 is connected to the oscillator 406. An output of the mixer 416 is connected to an input of an I/Q demodulator 418. An output of the I/Q demodulator 418 is connected to a first input of a comparator 420. A second input of the comparator 420 is connected to an output of a delay element 422. An output of the comparator 420 is connected to a second input of the I/Q modulator 402. An input of the delay element 422 is connected to the first input of the I/Q modulator 402. The decoupling means 412, the attenuator 414, the second mixer 416, the I/Q demodulator 418 and the comparator 420 define a feedback for determining the predistortion parameters.
In the following, the mode of operation of a transmitting means according to FIG. 4 will be described briefly. An I/Q signal, which is e.g. a message-carrying baseband signal comprising an I component and a Q component, is modulated onto a carrier signal by means of the I/Q modulator. In order to compensate distortions of the first mixer 404 and of the amplifier 408, the I/Q modulator carries out a predistortion of the I/Q signal in addition to the modulation. This is important especially when transmit signals with non-constant envelopes are used. The latter occur e.g. in cases in which amplitude-modulated instead of frequency-modulated signals are used so as to achieve a higher spectral efficiency of the modulation method. The non-constant envelope of the transmit signal causes in connection with the non-linearities of the first mixer 404 and of the amplifier 408 disturbances outside the useful frequency band. These disturbances are referred to as adjacent-channel emissions and should typically not exceed an application-specific limit value. The predistorted output signal of the I/Q modulator 402 is fed to the first mixer 404 in which the signal is up-converted with the aid of the oscillator 406. The up-converted signal is then amplified by the amplifier 408, e.g. a travelling wave tube, and sent to the antenna 410 and transmitted.
Part of the signal sent to the antenna 410 is previously tapped off by the decoupling means 412 and, for further processing, it is attenuated by the attenuator 414 so as to reverse the effect of the amplification of the amplifier 408. The tapped-off attenuated signal is fed to the second mixer 416 for down-conversion. The down-converted signal is fed to the I/Q demodulator so as to be demodulated into an I/Q signal. The demodulated I/Q signal now carries the information on the distortion caused in the original I/Q signal by the first mixer 404 and the amplifier 408. When this demodulated distorted I/Q signal is supplied to the comparator 420, the comparison between the original I/Q signal and the distorted I/Q signal will provide the information indicating what predistortion of the I/Q modulator has to be chosen so that the distortions caused by the first mixer 404 and the amplifier 408 can be compensated for in the best possible way.
A feature which is important to the comparison is that the original I/Q signal is delayed in time by the delay element 422 prior to the comparison in the comparator 420 so that the original I/Q signal is actually the signal which caused the predistorted I/Q signal. This method of adjusting the predistortion of the I/Q modulator 402 in dependence upon a comparison is referred to as adaptive predistortion.
An example of such an adaptive predistortion is described in U.S. Pat. No. 5,049,832. U.S. Pat. No. 5,049,832 discloses an amplifier linearization of an amplifier circuit by adaptive predistortion in the case of which an input signal for a power amplifier of the amplifier circuit is derived from an input modulation signal of the amplifier circuit by predistortion, i.e. the input signal of the power amplifier is predistorted so as to achieve a linear amplification of the input signal by the power amplifier.
The predistortion of the input modulation signal is adjusted via a table, which is addressed in dependence upon the square of the amplitude of the input modulation signal, the contents of the table being continuously updated so that, when the input modulation signal is being distorted, variations of the distortion caused by the power amplifier can be taken into account through the table.
FIGS. 5A and 5B show the components and FIG. 5C shows the overall configuration of a conventional I/Q modulator 500 with predistortion of the I/Q signal or baseband signal.
FIG. 5A shows means 502 for applying an I/Q signal or baseband signal, which comprises an I component and an Q component, to a carrier signal, which comprises a first subcomponent, in this case a cosine component, and a subcomponent, in this case a sine component, which is substantially orthogonal to this first subcomponent, so as to produce an output signal y(t).y(t)=i(t)·cos ω0t−q(t)·sin ω0t with ω0=2πf0  equa. 1
Means 502 for applying an I/Q signal to a carrier signal comprises a first multiplier 506, a second multiplier 508, means 510 for producing a carrier signal and an adder 512.
The first multiplier 506 comprises a first input, which is a first input of the means 502 for applying an I/Q signal to a carrier signal and which has the I component of the I/Q signal applied thereto, a second input, which is connected to a first output of the means 510 for producing a carrier signal, and an output which is connected to a first input of the adder 512.
The second multiplier 508 comprises a first input, which is a second input of the means 502 for applying an I/Q signal to a carrier signal and which has the Q component of the I/Q signal applied thereto, a second input, which is connected to a second output of the means 510 for producing a carrier signal, and an output which is connected to an inverting second input of the adder 512.
The means 510 for producing a carrier signal produces a carrier signal which can be represented as a complex function in the following way:ejω0t=cos ω0t+j sin ω0t  equa. 2
The first multiplier 506 multiplies the first subcomponent of the carrier signal by the I component of the I/Q signal, as can be seen from the first multiplication of equation 1, so as to obtain a multiplied I component, and the second multiplier 508 multiplies the second subcomponent of the carrier signal by the Q component of the I/Q signal so as to obtain a multiplied Q component, as can be seen from the second multiplication of equation 1.
The adder 512 forms subsequent to the first multiplier 506 and the second multiplier 508 the difference between the multiplied I component and the multiplied Q component, as shown in equation 1, so as to produce the output signal y(t) of the means 502 for applying an I/Q signal to a carrier signal.
Also the I/Q signal is now represented as a complex function.x(t)=i(t)+jq(t)  equa. 3
The function of the means 502 for applying an I/Q signal to a carrier signal can be described by the following complex representation:y(t)=Re{x(t)·ejω0t}  equa. 4
FIG. 5B shows a predistortion means 504 for predistorting an I/Q signal, i.e. an I component and a Q component of an I/Q signal. In a complex representation, the I/Q signal is predistorted by multiplication with a predistortion signalp(t)=p1(t)+jp2(t)=ρ(t)·ejφ(t)  equa. 5so as to obtain a predistorted I/Q signal.xp(t)=x(t)·p(t)=ip(t)+jqp(t)  equa. 6ip(t)=i(t)·p1(t)−q(t)·p2(t)  equa. 7qp(t)=i(t)·p2(t)−q(t)·p1(t)  equa. 8
The predistortion means 504 comprises a first multiplier 514, a second multiplier 516, a third multiplier 518, a fourth multiplier 520, a first adder 522, a second adder 524, and means 526 for producing a predistortion signal.
The first multiplier 514 comprises a first input, which is connected to a first input of the predistortion means 504 and which has the I component of the I/Q signal applied thereto, and a second input, which is connected to a first output of the means 526 for producing a predistortion signal p(t) and which has applied thereto the first predistortion component p1(t) of the predistortion signal according to equation 5, and an output which is connected to a first input of the first adder 522.
The second multiplier 516 comprises a first input, which is connected to a second input of the predistortion means 504 and which has the Q component of the I/Q signal applied thereto, and a second input, which is connected to a second output of the means 526 for producing a predistortion signal and which has the second predistortion component p2(t) of the predistortion signal p(t) applied thereto, and an output which is connected to an inverting second input of the first adder 522.
The third multiplier 518 comprises a first input, which is connected to the second input of the predistortion means 504 and which has the Q component of the I/Q signal applied thereto, a second input connected to the first output of the means 526 for producing a predistortion signal, and an output connected to a first input of the second adder 524.
The fourth multiplier 520 has an input, which is connected to the first input of the predistortion means 504 and which has the I component of the I/Q signal applied thereto, a second input connected to the second output of the means 526 for producing a predistortion signal, and an output connected to a second input of the second adder 524.
An output of the first adder is a first output of the predistortion means 504 and an output of the second adder is a second output of the predistortion means 504.
The means 526 for producing the predistortion signal supplies at the first output the first component p1(t) of the predistortion signal p(t) and at the second output the second component p2(t) of the predistortion signal p(t) depending on the I component i(t) of the I/Q signal and on the Q component q(t) of the I/Q signal, the I component being applied to a first input of the means 526 for producing the predistortion signal and the Q component of the I/Q signal being applied to a second input.
In the following, the mode of operation of the predistortion means 504 shown in FIG. 5B will be described briefly. The first adder 522 has the function of forming the difference indicated in equation 7, the first multiplier 514 carrying out the first multiplication occurring in equation 7 and the second multiplier 516 carrying out the second multiplication occurring in equation 7. The second adder 524 has the function of forming the sum indicated in equation 8, the third multiplier 518 carrying out the second multiplication occurring in equation 8 and the fourth multiplier 520 carrying out the first multiplication occurring in equation 8.
FIG. 5c shows the overall configuration of the conventional I/Q modulator 500 with predistortion of the I/Q signal comprising the means 502 for applying an I/Q signal to a carrier signal according to FIG. 5A and the predistortion means 504 according to FIG. 5B.
The conventional I/Q modulator 500 according to FIG. 5C now supplies at its output, i.e. as a result of the addition of the adder 512, the following output signal:y(t)=Re{x(t)·p(t)·ejω0t}  equa. 9y(t)=Re{xp(t)·ejω0t}=ip(t)·cos ω0t−qp(t)·sin ω0t  equa. 10This is the predistorted I/Q signal modulated on a carrier signal.
FIG. 6 shows an I/Q modulator 600 with predistortion of the carrier signal. In contrast to the I/Q modulator according to FIGS. 5A, B, C, the carrier signal, instead of the I/Q signal, is now predistorted by a predistortion signal p(t) so as to obtain a predistorted carrier signal.tp(t)=p(t)·ejω0t=ρ(t)·ej[ω0t+φ(t)]  equa. 11
For an I/Q modulator with predistortion of the carrier signal, the output signal of this I/Q modulator is obtained on the basis of equation 9 and equation 11.
                                                                        y                ⁡                                  (                  t                  )                                            =                              Re                ⁢                                  {                                                                                    x                        _                                            ⁡                                              (                        t                        )                                                              ·                                                                                            t                          _                                                p                                            ⁡                                              (                        t                        )                                                                              }                                                                                                        =                                                                    i                    ⁡                                          (                      t                      )                                                        ·                                      ρ                    ⁡                                          (                      t                      )                                                        ·                                      cos                    ⁡                                          [                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  ϕ                          ⁡                                                      (                            t                            )                                                                                              ]                                                                      -                                                      q                    ⁡                                          (                      t                      )                                                        ·                                      ρ                    ⁡                                          (                      t                      )                                                        ·                                      sin                    ⁡                                          [                                                                                                    ω                            0                                                    ⁢                          t                                                +                                                  ϕ                          ⁡                                                      (                            t                            )                                                                                              ]                                                                                                                              equa        .                                  ⁢        12            
The I/Q modulator 600 in FIG. 6 comprises a first multiplier 602, a second multiplier 604, a third multiplier 606, a fourth multiplier 608, means 610 for producing a carrier signal, an adder 612 and means 614 for producing a predistortion signal.
The first multiplier 602 comprises a first input, which is connected to a first input of the I/Q modulator 600 and which has the I component i(t) of the I/Q signal applied thereto, a second input connected to an output of the second multiplier 604, and an output connected to a first input of the adder 612.
The second multiplier 604 comprises a first input, which is connected to a first output of the means 614 for producing a predistortion signal and which has the magnitude ρ(t) of the predistortion signal p(t) applied thereto, and a second input, which is connected to a first output of the means 610 for producing a carrier signal and which has a first subcomponent of the carrier signal, here a cosine function, applied thereto, and the output which is connected to the second input of the first multiplier 602.
The third multiplier 606 comprises an input, which is connected to a second input of the I/Q modulator 600 and which has the Q component q(t) of the I/Q signal applied thereto, a second input connected to an output of the fourth multiplier 608, and an output connected to an inverting second input of the adder 612.
The fourth multiplier 608 comprises a first input, which is connected to a first output of the means 614 for producing a predistortion signal and which has the magnitude ρ(t) of the polar predistortion signal p(t) applied thereto, a second input, which is connected to a second output of the means 610 for producing a carrier signal and which has a second subcomponent of the carrier signal, here the sine function, applied thereto, and the output which is connected to the second input of the third multiplier 606.
The means 610 for producing a carrier signal comprises the above-mentioned first and the above-mentioned second output, which have applied thereto the first and second subcomponents of the carrier signal, here the cosine and sine components, and an input, which is connected to a second output of the means 614 for producing the predistortion signal and which has the phase φ(t) of the polar predistortion signal applied thereto.
Depending on at least the I component i(t) of the I/Q signal at a first input of the means 614 for producing a predistortion signal, which is connected to the first input of the I/Q modulator 600, and the Q component q(t) of the I/Q signal at a second input of the means 614 for producing a predistortion signal, which is connected to the second input of the I/Q modulator 600, the means 614 for producing a predistortion signal supplies at the first output thereof the magnitude ρ(t) of the predistortion signal p(t) and at the second output thereof the phase φ(t) of the predistortion signal p(t).
In the following, the mode of operation of the I/Q modulator 600 with predistortion of the carrier signal according to FIG. 6 will be described briefly. The adder 612 has the function of forming the difference in equation 12. The first summand of equation 12 is produced by the first multiplier 602 and the second multiplier 604 and the second summand of equation 12 is produced by the third multiplier 606 and the fourth multiplier 608.
The second multiplier 604 performs the second multiplication of the first summand of equation b 12, i.e. the multiplication of the first subcomponent of the carrier signal with the magnitude of the predistortion signal, so as to produce a predistorted first subcomponent of the carrier signal, whereas the first multiplier 602 performs the first multiplication of the first summand of equation 12, i.e. the multiplication of the predistorted first subcomponent of the carrier signal with the I component of the I/Q signal.
The fourth multiplier 608 performs the second multiplication of the second summand of equation 12, i.e. the multiplication of the second subcomponent of the carrier signal with the magnitude of the predistortion signal, so as to obtain a predistorted second subcomponent of the carrier signal, and the third multiplier 606 performs the first multiplication of the second summand of equation 12, i.e. the multiplication of the predistorted second subcomponent of the carrier signal with the Q component of the I/Q signal.
One disadvantage of the conventional I/Q modulator 500 with predistortion of the I/Q signal according to FIGS. 5A, B, C, and of the I/Q modulator 600 with predistortion of the carrier signal according to FIG. 6 is that six and four multipliers, respectively, are required for realizing the I/Q modulators in circuitry. In the case of modern transmitting means the predistortion and the I/Q modulations are carried out digitally i.e. in a time-discrete manner. In view of the large bandwidth and the high precision demands of modern transmission methods, such as e.g. W-CDMA (CDMA=Code-Division Multiple Access), expensive, fast, digital multipliers having a high resolution of typically 14 bits are required for this purpose.
Another disadvantage is that, in view of the high numbers of multipliers, the number of gates and the power consumption of the conventional I/Q modulators according to FIGS. 5A, 5B, 5C and according to FIG. 6 are very high.